This invention relates to a process for controlling operation of a fuel-operated heater, especially of a vehicle auxiliary heater and to a control arrangement for this purpose.
In heaters, which have air as a heat transfer medium, such as that of U.S. Pat. Nos. 4,530,658 and 4,685,616 as well as German Offenlegungsschrift No. 35 09 349, the temperature in the heat exchanger determines the value of the heating power that can be reached with the heater without damaging the components. In this case, the temperature in the heat exchanger is dependent on the temperature and amount of air drawn in, or water circulated, to be heated by the heat exchanger for production of the heating air. Since in the operation of such heaters, different air or water throughputs and temperatures as well as different fuel amounts can occur, so that for reasons of safety of the components, in the usual control and adjustment of the heater the nominal (rated) heating power of such a heater must be lowered in comparison with that heating power that could be delivered at maximum permissible heat exchanger temperature.
Control processes and devices for heating devices are known. U.S. Pat. No. 4,685,616 discloses one such process and a switching means for a heating device wherein a temperature gradient with respect to time is utilized as a governing value in controlling operating conditions of the heating device with a goal of achieving increased efficiency by coordinating heat supply to heat demand while also avoiding critical operating conditions. Furthermore, in allowed U.S. patent application Ser. No. 863,459, filed May 15, 1986, a heating device is disclosed having a control unit for controlling operation of the heating device which utilizes a pair of temperature sensors to constantly monitor operating conditions of the heat exchanger of the heating device (based upon a temperature differential occurring between the sensors) so as to detect operating conditions which could potentially cause an overheating condition to occur. In response to such a potential overheating causing condition being detected, operating conditions of the heating device are modified to prevent the device from actually overheating. However, such control systems are not designed to increase output performance (i.e., to increase performance above the theoretical, rated output, value) of a heating device.
The invention, therefore, aims at providing a process for operating a heater of the initially mentioned type, as well as a control arrangement for this purpose, which allows a more complete use of the deliverable heating power. That is, to provide a process for an arrangement which does more than merely reduce output when actual conditions prove excessive relative to designed for conditions, and will also increase output when actual conditions are less demanding than maximum designed for conditions.
For this purpose, a process for operating a fuel-operated heater of such a type is distinguished by the fact that the maximum instantaneously producible heating power is adjusted in accordance with the instantaneous operating condition of the heater. Therefore, in this operating process, an instantaneous operating condition of the heater, e.g., the load condition or the temperature at the most highly loaded spots of the heater, is taken as the control variable for the instantaneously maximum producible heating power. Thus, the process according to the invention makes it possible to always operate such a heater within the limiting power range without the components, for example, being endangered by overload. In this way, the heating power producible with the heater can be optimized and increased without the basic principles of such a heater having to be changed.
According to a preferred embodiment of the process according to the invention a characteristic component temperature is used as a control variable for determination of the instantaneous operating condition of the heater, and in this case, the temperature of the heat exchanger is suitable. Since, generally, the end of the heat exchanger, on which the hot combustion gases coming from the combustion chamber are deflected in an opposite direction, is most critical, the temperature at that end of the heat exchanger is considered as th e controlling value for the instantaneous operating condition in the heater.
Alternatively, the instantaneous operating condition of the heater can also be determined from the temperature of the heating air, since this temperature is in a relatively constant ratio to that of the temperature-critical areas of the heater.
According to the invention, the adjustment of the heating power takes place with the help of an adjustment of the amount of fuel that is fed to the burner. In this connection, it can be suitable to, additionally, adjust the amount of combustion air fed to the burner.
According to a second essential aspect according to the invention, a control arrangement for carrying out of the process is provided which has a detection device for detecting the instantaneous operating condition of the heater as well as an adjustment device for the instantaneous maximum producible heating power, which works with the detection device.
If, with such a control arrangement, a characteristic component temperature is detected as the controlling value for the instantaneous operating condition, a temperature sensor is preferably placed on the heat exchanger, namely on its end. Alternatively, the detection device can detect the temperature of the heating air, for example, with the help of a temperature sensor in the outgoing heating air current, to affect, in an appropriate way, the adjustment device for the instantaneous maximum producible heating power.
For the adjustment of the maximum producible heating power, preferably, an amount adjustment device is present which appropriately controls the amount of fuel fed to the burner in accordance with the detected operating condition. In addition, the amount of combustion air is also optionally adjusted.
Further, for the maximum producible heating power, the adjustment device can also be connected to a sensor for the heating air discharge temperature and/or for the ambient temperature so that these influencing values can be considered as reference values in the adjustment device to match the heating power that can be delivered by the heater to the actual heating requirements.
In summary, the process principle and arrangement according to the invention make it possible, in the case of heaters of a predetermined size, for the maximum producible heating power to be increased without danger to the components. Also the heating air temperature changes essentially less as a function of the temperature of the air drawn in. Further, this increased heating power is achieved in such a way that only insignificantly more electric power must be used for the operation. Simultaneously, the heating up time is shortened, since the heater ca n be operated with maximum and, according to the invention, with increased maximum heating power.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, a single embodiment in accordance with the present invention.